1. Field of the Invention
The present invention relates to a process for producing polycrystalline dense shaped bodies based on boron carbide by pressureless sintering.
2. The Prior Art
Boron carbide is a material which can be densified without pressure only by addition of sintering aids. DE 275 19 98 (corresponding to U.S. Pat. No. 4,195,066) teaches that suitable sintering aids are carbon-containing substances in the form of phenolic resins which are converted by pyrolysis to amorphous carbon, or else pure carbon in the form of carbon black. A prerequisite for good sinterability is from 2 to 6% by weight of carbon distributed as homogeneously as possible on the surface of the boron carbide powder particles.
Phenolic resins have particularly good qualifications for achieving this homogeneous distribution. In the production of dense bodies according to this process, a boron carbide slip provided with the necessary auxiliaries is first sprayed to form a granular material and this is made into a suitable shape by uniaxial pressing. The green shaped bodies are subsequently heat-treated at about 1000.degree. C., the phenolic resins being pyrolyzed so that finally each boron carbide powder is surrounded by a thin shell of amorphous carbon. Besides this process, the direct dispersion of carbon black is also a suitable process for introducing carbon as sintering aid if it is ensured that the distribution of the carbon black particles is sufficiently homogeneous.
Both of the sintering additives are associated with not inconsiderable process problems. Phenolic resins tend to stick in the uncarbonized state, considerably impairing the flow ability of the sprayed granular materials, so that the automatic filling of pressing dies can become problematic. In addition, boron carbide powders doped with phenolic resin "age" as a result of polymerization reactions, which is why these powders have to be stored at low temperatures.
Additions of carbon black, which do not show either of these disadvantages, can, in contrast, only be dispersed with difficulty, so that the sintering aid is not always present in sufficiently homogeneous distribution on the boron carbide surfaces, which finally leads to unsatisfactory sintered densities, i.e., to sintered bodies having open porosity (less than 92% TD). Such bodies cannot, without encapsulation, be further densified by hot isostatic pressing to densities greater than 99% of the theoretical density (TD). However, bodies having .rho. greater than 99% TD are a prerequisite for the usability of boron carbide in numerous applications.
In addition, in American Institute of Physics, Conference Proceedings Boron-Rich Solids, No. 213, pp. 464-467, Zakhariev and Radev describe a process for the pressureless sintering of boron carbide with the inclusion of a liquid phase. This process is based on mixing boron carbide with tungsten carbide and reacting this mixture with the formation of W.sub.2 B.sub.5 in accordance with EQU 5B.sub.4 C+8WC.fwdarw.13C+4W.sub.2 B.sub.5 ( 1)
Here, the action of W.sub.2 B.sub.5 in promoting sintering is utilized. W.sub.2 B.sub.5 is a sintering aid for boron carbide because the two substances form a quasibinary eutectic system with a eutectic temperature of about 2220.degree. C. The experiments of Zakhariev and Radev showed that at sintering temperatures above the eutectic temperature, i.e., at sintering temperatures above 2220.degree. C., sintered densities of greater than 90% TD are achieved. Below 2220.degree. C., i.e., without liquid phase, only densities of less than 82% TD were achieved (see FIG. 2 of the cited reference). Solid phase sintering of boron carbide, utilizing the sintering-promoting action of free carbon, which is formed by the addition of tungsten carbide and a subsequent reaction at temperatures below the sintering temperature in accordance with equation (1), is consequently not possible by the process described. The process according to Zakhariev and Radev, which is characterized by high sintering temperatures and the formation of liquid phases, greatly accelerates the grain growth of boron carbide, which is a disadvantage. Because of the anisotropy of boron carbide, coarse-grained microstructures causes high residual stresses and finally cracks, so that boron carbide, bodies having a coarse microstructure are unusable for practical applications.